Process for producing pigment-quality titanium dioxide

ABSTRACT

An axial-type reactor for producing pigment-quality titanium dioxide is disclosed which comprises a reaction chamber and a plurality of spaced-apart concentric tubes which define a plurality of concentric passages for introducing at their exit end into the upstream end of the reaction chamber gaseous TiCl4, O2 and CO, and wherein the tubes which define the passage for TiCl4 protrude into said reaction chamber a greater distance than the other tubes, thereby locating the exit end of said TiCl4 passage downstream with respect to the exit ends of the other passages. The corresponding process is also disclosed.

United States Patent 1 1 3,764,667 Thring et al. Oct. 9, 1973.

[ PROCESS FOR PRODUCING 2,980,509 4/1961 Frey 23/202 v PIGMENT QUALITY TITANIUM DIOXIDE 3,355,253 11/1967 Tillmann et al.... 23/202 V X 3,360,341 12/1967 Tillmann et al.... 23/202 V X Inventors: Meredith Wooldrige g, 3,365,274 1/1968 Carpenter et al.. 23/202 v Buckhurst Hill, England; Dante 3,518,052 6/1970 Foulds 23/202 V Cadorin; Aldo Ducato, both of Novara Italy Primary ExaminerEdward Stern [73] Assignee: Montecatini Edison S.p.A., Milan, Attorney-Stevens, Davis, Miller & Moshef Italy [22] Filed: May 7, 1971 57 ABSTRACT [21] Appl' 141366 An axial-type reactor for producing pigment-quality titanium dioxide is disclosed which comprises a reac- 30 Foreign Application Priority m tion chamber and a plurality of spaced-apart concen- M ay H 1970 Italy 4406 M70 tric tubes which define a plurality of concentric pas- I sages for introducing at their exit end into the up- 52 US. Cl u 4 3 6 3 4 3 3 7 .stream end of the reaction chamber gaseous TiCl 0 Int CL 2 l 2 and CO, and wherein the tubes which define the pas- [58] Field V 1 1 B sage for TiCL, protrude into said reaction chamber a greater distance than the other tubes, thereby locating the exit end of said TiCl passage downstream with re- [56] References Cited spect to the exit ends of the other passages. The corre- UNITED ST ATE S PATENTS sponding process is also disclosed.

2,779,662 1/1957 Frey 23/202 V 2 Claims, 4 Drawing Figures LO [Q 7 08 I 42 12 \9 I 7 /4- 1/ x /5 l 2 II 17 l [4 A r j 7 T 1 9 4 s 6 A 3 I t A I3 A a PATENTEU 91975 3.764.667

F/GJ

INVENTORS MEREDITH WOOLDRIDGE THRING,

DANTE CADORIN,

ALDO DUCATO nm pm M81 m ATTORNEYS PROCESS FOR PRODUCING PIGMENT-QUALITY TITANIUM DIOXIDE The present invention relates to an improved reactor and process for producing finely divided metal oxides. More particularly, the invention relates to a reactor and to a process for producing pigment-quality titanium dioxide, by reacting in gaseous phase atitanium halide with oxygen or with an oxygen-containing gas. Still more particularly, the present invention relates to an improved axial-type reactor for producing pigmentquality titanium dioxide and'to the corresponding process.

As is well known, an axial-type reactor is a substantially cylindrical reactor in which the reagent streams are fed into the mixing and reaction zone while flowing in the same, and substantially parallel, directions withrespect to the longitudinal axis of the reactor. It' is well known that it is possible to obtain finely divided'met'al oxides by oxidizing metal halides in gaseous phase. Usually, the reaction temperature is higher than 900C. More particularly, it iswell-known to produce pigmentquality titanium dioxide byreacting TiCl} with oxygen or with an oxygen-containing gas (usually air) in accordance with the following reaction:

The chlorine thus obtained as a byproduct can be used methods most widely used for-thatpurpose are: indirect heating of the reactants up to about 8001000C in suitable chambers lined with inert materials, such as for instance silica, amorphous carbon or'graphite, before feeding them in the reactor; or direct heating by means of an auxiliary flame obtained by burning a mixture'of carbon monoxide (or'some other fuel) and-oxygen.

Axial-type reactors for producing pigment-quality titanium dioxide have been broadly known for a long time. Such reactors usually comprise a reaction chamber and a plurality of co-axial, generally cylindrical, spaced-apart tubes which define a plurality of annular co-axial passages, through which and from which the various gaseous reactants are fed into the reactionchamber. For instance, an axial reactor has been recently disclosed in which the downstream ends of various tubes, i.e., those ends which terminate in the reaction chamber, are coplanar. The reactants are separately fed according to the following order from the central innermost passage to the annular outermost one: oxygen, carbon monoxide, titanium tetrachloride", carbon monoxide and oxygen. Such an arrangement of reactants has the purpose of avoiding; an untimely or premature reaction between the titanium tetrachloride and the oxygen in theimmediate neighborhood of the downstream ends of the tubes, which would cause the deposition thereon of solid reaction products. Such deposits might in turn stop the smooth running and eveneventually lead to the complete pluggingof the reactor; In order to minimize the occurrence of such deposits, it has been suggested to interpose a stream of an inert gas, e.g., nitrogen, between the concentric streams of titanium tetrachloride and carbon monoxide. However, the use of inert gases presents certain drawbacks. For instance, as has been mentioned before, the chlorine produced in the Tic], -oxidization step is used in the step of chlorinating the starting titanium-bearing material in order to obtain additional TiCl The addition of an inert gas therefore leads to the dilution of the byproduct chlorine thus obtained. This is obviously an undesirable result, since it then becomes necessary to provide larger equipment thanwould be required if it were possible to'obtain and utilize undiluted chlorine.

Furthermore, in none of the axial reactors and related processes disclosed so far, which utilize an auxiliary flame for heating the reactants, has it been possible to avoid that the oxidation reaction of TiCl, takes place in a carbon monoxide-free environment. The presence of unburnt carbon monoxide during the reaction between titanium tetrachloride andoxygen causes many drawbacks. For instance, it is well known that TiCl, is a-strongcombustioninhibitor and therefore the carbon monoxide which is present in the zone of mixing the TiClg, with 02 cannot be thoroughly burnt. Therefore a lower mixing temperature and a slower starting of the oxidation reaction are obtained. Furthermore, carbon monoxide promotes the re-chlorination of TiO, particles, thus causing a decrease in the reaction yield andthe obtaining of a TiO product consisting of particles'much too uneven in size.

In accordance with the present invention, an improved process has been discovered for producing an excellent pigment-quality titanium dioxide while overcoming the drawbacks of the prior art processes. Furthermore, animprovedaxial-flow reactor for carrying out the process has been devised as a part of the invention.

Thus, in accordance with this invention the process for producing pigment-quality titanium dioxide by reacting titanium tetrachloride with oxygenin thepresence of an auxiliary flame obtained by the combustion of a carbonaceous gaseous fuel (e.g. carbon monoxide) into-carbon dioxide, substantially comprises feeding gaseous co-axial and separate streams of titanium tetrachloride, oxygen and gaseous'fuel into a reaction zone and is characterized in that the titaniun tetrachloride stream is fed into that part of the reaction zone in which only oxygen and carbon dioxide are present which carbon dioxide results from the previously-.

completed combustion of the carbonaceous fuel.

In accordance with one of the main features of the invention, the gaseous titanium tetrachloride stream is introduced into the reaction zone in the form of a substantially annular stream. Such a stream is surrounded both outwardly and inwardly by two streams consisting essentially of a mixture of oxygen and carbon dioxide.

The improved axial-type reactor by means of which the process is carried out, and which embodies other features of the invention, comprises a reaction chamber and a plurality of concentric spaced-apart tubes which define a plurality of concentric passagesfor introducing at their exit ends into the reaction'chamber the titanium tetrachloride, the oxygen and the gaseous carbonaceous fuel.

More particularly, the reactor is characterized in that the tubes which define the TiCh-passage protrude into the reaction chamber a greater distance than the other tubes, whereby the exit end of said TiCl, -passage is located downstream with respect to the exit ends of the other passages;

In accordance with a preferred embodiment, said concentric tubes are cylindrical and define a plurality of annular peripheral passages plus a central cylindrical passage.

The process and reactor of the present inventionwill be illustrated as to further details by reference to the accompanying drawings in which:

FIG. 1 is a longitudinal sectional view of a reactor constructed in accordance with the present invention.

FIG. 2 is a sectional view taken along line A-A of FIG. 1.

FIG. 3 is a longitudinal sectional view of another reactor constructed in accordance with this invention.

FIG. 4 is a sectional view taken along line A-A of FIG. 3.

Referring now to the drawings, and particularly to FIGS. 1 and 2, reactor R comprises cylindrical and coaxial tubes 7, 8, 9, 10, I1 and 12 which are numbered from the outermost to the innermost one. These tubes define a plurality of co-axial passages suitable for introducing various reactants into the reaction chamber 13, having diameter d.

Reaction chamber 13 comprises a CO-combustion chamber 13' and a Tick-reaction chamber 13", the latter extending downwardly from the exit ends of tubes and 11. TiCL-reaction chamber, 13", has a length, l, which is preferably between about five and times the diameter d. The overall length of reaction chamber 13 (namely 13' 13") is generally between about 5.3 and 15.6 times the diameter d of the reaction chamber.

In accordance with the most characteristic feature of this invention, tubes 10 and 11 which define passage 1 for introducing TiCl, into the reaction chamber 13, protrude into the reaction chamber so that their exit ends extend below the exit ends of tubes 8, 9 and 12 for a distance, 4, which is generally between about 0.3 and 0.6 times the diameter d of the reaction chamber.

The thickness, b, of passage 1 is preferably between about 0.01 and 0.! times the diameter d.

Diameter, a, of tube 11 (i.e. the innermost of the tubes which define the TiCl -passage) is preferably between about 0.4 and 0.6 times the diameter d of reaction chamber 13.

In order to avoid the occurrence of vortices, tubes 10 and 11 are preferably tapered at their exit ends. The overall taper angle, a is generally between about 4 and The TiCl, is usually preheated up to a temperature of about 400 500C before being fed to the reactor. The feed velocity of the TiCl, is between about 20 and 120 m/s (meters per second).

Theinner wall of tube 7 and the outer wall of tube 8 define passage 2 in which an oxygen stream, generally preheated up to about 750 800C, is fed. The feed velocity of the oxygen is between about 10 and 40 m/s.

The inner wall of tube 8 and the outer wall of tube 9 define passage 14 in which a CO-stream, generally preheated up to about 380 420C, is fed.

The inner wall of tube 9 and the outer wallof tube 10 define passage 3 in which an O -stream,"prehe ated at the same temperature as that flowing along passage 2, is fed. The feed velocity of the oxygen is betwee about 10 and 40 m/s.

The inner wall of tube 11 and the outer wall of tube 12 define passage 4 in which an oxygen stream, preheated to the same temperature as the oxygen flowing along passages 2 and 3, is fed. The feed velocity of'the oxygen is between about 10 and 40 m/s. i i

Carbon monoxide, preheated to the same temperature as that flowing along passage 14, is fed into the central tube 12. The feed velocity of the CO into the CO-combustion chamber, 13', is between about 40 and m/s.

The inner walls of the outermost tube 7 extending downwardly from the ends of tubes 10 and 11 define the TiCL-reaction chamber, 13".

In accordance with a preferred embodiment of this invention, tubes 8 and 9 are joined at their exit ends by means of an apertured metal annulus 15, as clearly shown in FIG. 1. Holes 5 made in the metal annulus 15 cause the carbon monoxide to flow from passage 14 to the CO-combustion chamber below, 13', in the form of thin separate streams. Preferably, the holes 5 are located along a circumference of the center line of the metal annulus 15. Still in accordance with this preferred embodiment of the invention, the central tube 12 which defines the cylindrical central passage has at its exit end an apertured diaphragm 16 for causing the CO to flow into the combustion chamber 13 in the form of thin separate streams. The feed velocity of the CO into the combustion chamber 13 is between about 40 and 90 m/s. Preferably, holes 6 of the diaphragm 16 are located along a circumference which is concentric with respect to the axis of tube 12 see FIG. 2.

The purpose of having the CO-feed subdivided into a plurality of thin gaseous streams which is surrounded both inwardly and outwardly by two annular oxygen streams that are both co-axia] with respect to the reactor is that, in this way, the CO-combustion reaction is highly stabilized. Furthermore, the auxiliary flame is steadily anchored to the burner.

Referring now to FIGS. 3 and 4, the reactor T which is a modification of the reactor R shown in FIGS. 1 and 2 comprises the same structural elements 1 through 16, the difference here being that the contiguous tubes 10 and 11 which define the annular TiCl passage are joined at their exit ends by means of an apertured metal annulus 17, for causing the TiCl, to flow into the reaction chamber in the form of thin separate streams. For instance, there may be from 10 to 40 rectangular holes constituting the apertures in the annulus l7 and having dimensions b and q, q being preferably between about one and three times b. In this arrangement the feed velocity of the TiCl, is preferably between about 50 and m/s. In any event, the velocity of the reaction mixture inside the reaction chamber 13 is between about 30 and 90 m/s.

Although in accordance with the foregoing preferred embodiments of the reactor of this invention the exit ends of tubes 8, 9 and 12 are coplanar, modified embodiments in which said exit ends are not coplanar are also regarded as being within the scope of this invention. However, in such case, the various distances h, h h, etc. of the exit ends of the tubes which define, the Tick-passage and the exit ends'of other passages must still be within the above-mentioned range, that is between about 0.3 and 0.6 times the diameter d of the reaction chamber 13.

The axial-flow reactors made in accordance with this invention are buiit from high temperature-and-corrosion-resistant alloys. For instance, one may advantageously employ inox steels 16-25-6 (Cr-Ni-Mo), cobalt alloys such as Haynes, and nickel alloys such as Nimonic, Inconel and l-Iastelloy. The walls of the reactors (both in the combustion and the reaction zones) are cooled, for instance with pressurized water or with high-boiling diathermic oils or other well known cooling agents, for maintaining the walls at temperatures varying between 300 and 500C.

The titanium dioxide obtained by means of the reactor and process of the present invention is endowed with excellent pigment characteristics. It is substantially all made up of the rutile form of titanium dioxide and has a narrow distribution of particle dimensions. The average particle size is from about 0.18 to 0.25 microns. The percent number coefficient of variation ranges between about 25 percent and 40 percent.

The measure of the average particle size employed herein is the arithmetic mean", d which is defined as follows:

2 ifl wherein:

n is the total number of particles measured (in the experimental data, n 2,000); d, is the diameter of a given particle; and f, is the frequency, i.e. the number of particles having diameter d, This arithmetic mean is defined, for instance, in Particle size Theory and Industrial Applications By Richard D. Cadle, Reinhold Publishing Co., New York 1965, the relevant part of which is incorporated herein by reference.

As a measure of the particle size distribution herein is meant the percent number coefficient of variation. By percent number coefficient of variation (hereinbelow simply referred to as v) is meant the following percent'ratio:

v sli x 100 wherein:

s is the standard deviation of a distribution, equal to the square root of the variance.

(The variance of a distribution is equal to the mean of the squares of the deviations from the mean).

As is well known, the standard deviation is a measure of the average spread of the distribution.

3 is the above-mentioned arithmetic mean.

This percent number coefficient of variation is defined, for instance, in Statistical methods in research and production with special reference to the chemical industry" by Owen L. Davies, Oliver and Boyd, London 1963, the relevant part of which is incorporated herein by reference.

The pigment obtained in accordance with the present invention furthermore shows an excellent decoloration power. The values of tinting strength were determined by the New Jersey Zinc method described in the book by H.A. Gardner and 6.6. Sward, Paints, Varnishes, Lacquersand Colors, Gardner Lab. Inc., Bethesda, Maryland, U.S.A. (Page 52, 12th Edition, March 1962). Regular tinting strength is measured using one gram of TiO One gram of TiO is mixed with 0.82 grams of castor oil and with a determined amount of black pigment (made up of one part by weight of black powder and seven parts by weight of calcium carbonate): the homogenized sample thus obtained is compared with a standard sample. The amount of black pigment neededin order to obtain the equivalence between the tested sample and the standard sample, gives the measure of the tinting strength. The test called dilute tinting strength" is carried out in a similar manner,

while starting with a mixture of 0.15 grams of TiO: and 0.85 grams of calcium carbonate. The difference between the regular tinting strength and the dilute tinting strength, and the tone of the grey mixture gives information on the pigment grain.

The following detailed working examples are given still further to illustrate the present invention.

EXAMPLE 1 The reactor shown in FIGS. 1 and 2 was employed. Its characteristics were as follows (the values given are in millimeters): d= 61; l= 800; a 32; h 27; b 2.

A total of 25 Nm lh of oxygen preheated to 750C were fed to the reactor via passages 2, 3 and 4. A total of 12 Nm lh of carbon monoxide preheated to 400C were fed to the reactor via passage 14 and the tube 12.

After the CO-combustion reaction had been started and had reached thermal equilibrium, 138 kg/h of TiCl, preheated to 500C (and containing 1 percent by weight of AlCl as rutilizing agent) were fed into the reactor.

The reaction temperature was 1530 i 20C. After a l0hour run, 550 Kg of pigment-quality TiO, (rutile) were obtained. The average particle size was 0.21 microns. The percent number coefficient of variation was 35 percent. The Regular Tinting Strength (hereinbelow simply referred to as RTS) was 1620 blue tone 1. The Dilute Tinting Strength value (hereinbelow simply referred to as DTS) was 1730 blue tone 2.

EXAMPLE 2 The same type ofreactor was used as in Example 1. The relevant characteristics were the same as in Example 1, except that here h was decreased to 10 millimeters instead of 27. Under such conditions, the combustion reaction of the CO was not completed before the burnt gases became mixed with the TiCl.,. The CO combustion yield was detected by withdrawing some samples of burnt gases by means of cooled probes. Such withdrawals were effected near the TiCl, exit slot, before feeding in the TiCl A total of 30.5 Nm /h of oxygen preheated to 750C and a total of 15 Nm /h of CO preheated to 400C were fed into the reactor, under the same conditions as in Example 1. After starting the CO-combustion reaction and reaching thermal equilibrium, 173 Kg/h of TiCl, preheated to 500C and containing 1 percent by weight of AlCl were fed into the reactor.

The reaction temperature was 1540 i 20C. After a 15- hour run, 1050 Kg of pigment-quality titanium dioxide (rutile) were obtained. The average particle size was 0.17 microns. The percent number coefficient of variation was 40 percent. The RTS value was 1530 blue tone 2. The DTS value was 1580 blue tone 2.

EXAMPLE 3 The reactor shown in FIGS. 3 and 4 was employed. The relevant characteristics were as follows (the values given are in millimeters): d 61; l= 800; a 32; h 27; b 2; q 2.5; number of TiCl, jets 25.

A total of 17 Nm h of oxygen preheated to 750C were fed to the reactor via the passages 2, 3 and 4. A total of 7.5 Nm h of carbon monoxide preheated to 400C were fed to the reactor via the tube 12 and the passage 14.

After starting the CO-combustion reaction and reaching thermal equilibrium, 87 Kg/h of TiCl preheated to 500C and containing 1 percent by weight of AICI were fed into the reactor.

The reaction temperature was 1500 :t 20C. After an 18-hour run, 640 Kg of pigment-quality TiO, (rutile) were collected. The average particle size was 0.18 microns. The percent number coefficient of variation was 40 percent. RTS value was i580 blue tone 2; DTS value was 1640 blue tone 2.

The main features of the present invention and the advantages thereof may be summarized as follows:

a. The TiCh-stream is fed into a zone of the reaction chamber wherein no appreciable amounts of unburnt carbon monoxide exist; in fact the TiCl stream from the annular feed slot is surrounded, both inwardly and outwardly, by a mixture of excess oxygen and the carbon dioxide produced from the substantially complete combustion of CO. Consequently the drawbacks mentioned earlier are completely avoided.

b. By keeping steady the TiCl, feed rate and the thickness of the TiCl; annular slot, it is possible to scale up or down the reactor within wide limits without affecting the mixing time of the gaseous streams or the pigment characteristics.

c. The reactor in accordance with this invention is a piston-flow-type of reactor, i.e. without any recirculation. Therefore, the TiO,-particles obtained thereby have substantially the same history. This obviously makes it easier to obtain a uniform product endowed with a very high dimensional evenness,

!. It is possible readily to change the Tio -particles dimensions, simply by varying the thickness of the TiCL, annular slot.

Many modifications and variations may, of course, be

made without departing from the spirit and scope of the invention.

What is claimed is:

l. A process for producing pigment-quality titanium dioxide by reacting TiCl, and 0,, at about l480-l 560C, by means of an auxiliary flame obtained by complete combustion of CO to CO wherein coaxial and separate streams of gaseous TiCl O and C0 are fed into a reactor comprising a reaction chamber and a plurality of concentric spaced-apart tubes which define a plurality of annular concentric passages plus a central passage for feeding said reactants at their exit ends into the upstream end of said reaction chamber, characterized in that:

a. the TiCl the oxygen, and the carbon monoxide are respectively preheated to about 400-500C, to about 750800C, and to about 380420C;

b. the various gaseous reactants are fed according to the following order from the central passage to the outermost annular passage: carbon monoxide, oxygen, titanium tetrachloride, oxygen, carbon monoxide, oxygen; and

c. the TiCl, is fed into its corresponding passage, said passage protruding downwardly from other passages for feeding the other reactants by a distance which is between about 0.3 and 0.6 times the diameter of the reaction chamber, for introducing said TiCl, into that zone of the reaction chamber wherein only oxygen and carbon dioxide are present.

2. The process of claim 1, wherein the feed velocities of the various reactants from the corresponding passages are as follows:

the oxygen feed velocity is between about 10 and 40 m/s; the carbon monoxide feed velocity is between about 40 and m/s; and

the titanium tetrachloride feed velocity is between about 20 and m/s. 

2. The process of claim 1, wherein the feed velocities of the various reactants from the corresponding passages are as follows: the oxygen feed velocity is between about 10 and 40 m/s; the carbon monoxide feed velocity is between about 40 and 90 m/s; and the titanium tetrachloride feed velocity is between about 20 and 120 m/s. 